Allow to use relative dates in the format (now|today)[+-][0-9](day|week|month|year)(s)? (Closes #137)

Also fix DateRange not accepting ranges of one day.
Document the video selection using the upload date
Automatically set HTTPS proxy if given (Fixes #805)
Adapt tests to changes in youtube's "Most Popular" channel
release 2013.04.27
 Split FileDownloader in two classes: FileDownloader and YoutubeDL

YoutubeDL is the class that coordinates everything
FileDownloader gets a filename and an info dict and downloads the video.